The present invention relates to an objective lens for an optical head employed in an optical disc drive, which is capable of recording/readout data on/from various types of optical discs having different data densities and/or thickness of protective layers.
There are various standards for optical discs. Different optical discs according to different standards may have different data densities, different thickness of protective layers and the like. For example, a CD (compact disc) or a CD-R (compact disc recordable) has a relatively low data density, and the thickness of the protective layer is 1.2 mm. A DVD (digital versatile disc) has a relatively high data density, and the thickness of the protective layer is 0.6 mm.
For data recording/readout of the DVD, it is necessary to use a laser beam having a wavelength of 635-660 nm. For the CD-R, in view of its reflective characteristics, a laser beam having a wavelength of approximately 780 nm is to be used.
Both types of the discs are widely used, and therefore, it is preferable that an optical disc drive is capable of using both discs. Such a disc drive should be provided with two laser diodes which emit laser beams having different wavelengths, for the CD and DVD, as described above. In view of downsizing of the disc drive, it is preferable that an optical system for the disc drive is as compact as possible. In order to reduce the size of the optical system, the two laser diodes maybe incorporated in a single unit (i.e., a laser source unit), and further, an objective lens of the optical head may be configured to function for either of the laser beams having different wavelengths.
When the above-described laser source unit is used, however, the following problem arises.
Generally, two light emitting points of the two laser diodes may be aligned in a direction perpendicular to an optical axis of the objective lens. Therefore, there exists an angular difference between the beams incident on the objective lens. That is, at least one of the laser beams emitted by the two light emitting points is incident on the objective lens as an off-axial beam.
The optical system is preferably compensated with respect to aberrations for the two different laser beams having different wavelengths. However, since the thickness of the protective layers of the two discs are different, coma cannot be compensated completely. If, for example, the coma is not completely compensated with respect to a laser beam, which is used for one of the discs and incident on the objective lens as a off-axial light beam, a beam spot formed by the beam on the disc is not made sufficiently small, and therefore, data recording/readout on the disc may not be performed.
Further, a structure of a laser emitting device becomes larger as a required output is larger. If the structure is larger, a distance between the laser emitting points should also be longer. Accordingly, if the relatively high intensity is required in the above-described type optical head in order to improve a data recording/readout speed, the distance between the light emitting points becomes longer. In such a case, a difference between the incident angles of the laser beams with respect to the objective lens becomes larger. Therefore, at least one of the laser beams, which is incident on the objective lens as an off-axial beam, is incident on the objective lens at a relatively large incident angle. Then, relatively large aberrations are caused, and data recording/readout operation may not be performed.
It is therefore an object of the present invention to provide an improved objective lens for an optical disc drive employing a laser source unit accommodating two laser diodes emitting laser beams having different wavelengths. The objective lens is configured to suppress occurrence of aberrations, and forms appropriate beam spots on a plurality of optical discs based on different standards. The invention further provides an improved optical head provided with the improved objective lens, and optical disc drive employing such an optical head.
For the above object, according to the present invention, there is provided an objective lens for an optical head. The objective lens converges first and second laser beams on first and second optical discs, respectively. The first laser beam has a shorter wavelength than the second laser beam, and a protective layer of the first disc is thinner than a protective layer of the second optical disc. The objective lens is compensated for astigmatism with respect to the first beam, which is incident on the objective lens as an off-axis beam at a predetermined incident angle, by providing on-axis astigmatism corresponding to wave front aberration greater than or equal to 0.01xcex (rms), and the objective lens is configured such that the coma is compensated better in a case where the first laser beam is converged on the first optical disc than in a case where the second laser beam is converged on the second optical disc.
With this configuration, the wave front aberration of the objective lens when the first optical disc is used exhibits the maximum value when a beam is incident in parallel with the optical axis, and the minimum value when the beam is incident at predetermined incident angles (the same absolute value with positive/negative signs). Therefore, by adjusting the magnitude of the astigmatism so that the wave front aberration when the beam is incident in parallel with the optical axis does not exceed the allowable range, an range of the incident angle at which the wave front aberration does not exceed the allowable range, can be expanded.
Further, for the second optical disc, it is not necessary to form the beam spot so small as for the first optical disc. That is, the NA can be relatively small. In this case, the effect of the coma and the astigmatism by the off-axis beam onto the wave front aberration is relatively small. Therefore, even if the coma is not compensated, it is possible to enhance the range of the incident angle at which the wave front aberration does not exceed the allowable range.
When the above-described objective lens is used, if the shorter wavelength laser beam is incident on the objective lens at an incident angle at which the astigmatism has the minimum value for the first optical disc, it becomes possible to suppress the wave front aberration within an allowable range, for the second optical disc, even if the beam is not incident in parallel with the optical axis of the objective lens.
Optionally, the objective lens may be an infinite-conjugate system that receives the first and second laser beam as the parallel light fluxes and converges the received parallel light fluxes on the first and second optical discs, respectively.
Further optionally, the objective lens may be formed with a diffraction lens structure on one of refraction surfaces of the objective lens, the diffraction lens structure being configured such that spherical aberration is changed in an under corrected direction as a wavelength of a beam passing therethrough increases.
According to another aspect of the invention, there is provided an optical head for an optical disc drive, including a first laser diode that emits a first laser beam, a second laser diode that emits a second laser beam, a wavelength of the first laser beam being shorter than a wavelength of the second laser beam, and an objective lens that converges the first and second laser beams on first and second optical discs, respectively, a protective layer of the first disc being thinner than a protective layer of the second optical disc.
The objective lens is compensated for astigmatism with respect to the first beam, which is incident on the objective lens as an off-axis beam at a predetermined incident angle, by providing on-axis astigmatism corresponding to wave front aberration greater than or equal to 0.01xcex (rms).
Further, the objective lens is configured such that the coma is compensated better in a case where the first laser beam is converged on the first optical disc than in a case where the second laser beam is converged on the second optical disc.
Furthermore, the first laser diode and the second laser diode are arranged such that converging points, on which the first laser beam and the second laser beam are converged by the objective lens, are aligned in a direction in which one of two focal lines, which would be formed when light fluxes parallel with the optical axis of the objective lens are incident on the objective lens, closer to the objective lens extends.
Optionally, the first laser diode and the second laser diode are incorporated in a single unit.
Further, it is preferable that light emitting points of the first and second laser diodes are aligned at off-axis positions with respect to the optical axis of the objective lens.
According to a further aspect of the invention, there is provided an optical disc drive, which is provided with a spindle motor that is capable of rotating either one of a first optical disc and a second optical disc, an optical head and a head driving mechanism.
The optical head includes a first laser diode that emits a first laser beam, a second laser diode that emits a second laser beam, a wavelength the first laser beam being shorter than a wavelength of the second laser beam, and an objective lens that converges the first and second laser beams on first and second optical discs, respectively, a protective layer of the first disc being thinner than a protective layer of the second optical disc.
The objective lens is compensated for astigmatism with respect to the first beam, which is incident on the objective lens as an off-axis beam at a predetermined incident angle, by providing on-axis astigmatism corresponding to wave front aberration greater than or equal to 0.01xcex (rms).
Further, the objective lens is configured such that the coma is compensated better in a case where the first laser beam is converged on the first optical disc than in a case where the second laser beam is converged on the second optical disc. Furthermore, the first laser diode and the second laser diode are arranged such that converging points, on which the first laser beam and the second laser beam are converged by the objective lens, are aligned in a direction in which one of two focal lines, which would be formed when light fluxes parallel with the optical axis of the objective lens are incident on the objective lens, closer to the objective lens extends.
The head driving mechanism drives at least a part of the optical head in a radial direction of the one of the first and second optical disc. An extending direction of the focal line closer to the objective lens coincides with a tangential line to the one of the first and second optical disc.